" in fact i was under the impression that some cats could go 20 in 10knots of wind." - You`ll find cats or any sailing craft designed to acchieve this, will only do 10knots in 20knots of wind. As mentioned by others, the VX40`s can fly a hull in 4-6knots ? I`m assuming they are depowering drastically at 20knots, and tripping all over themselves in 25 plus, unless they reef sails. Unfortunately you can`t have a Lake Garda 5 knot specialist yacht and sail it comfortably in 30 knots, you have to choose the boat designed for a certain windrange, and sail within it.
I`ll agree with the sentiment that says that catamarans are at their most efficient in light to medium winds - you might sail at 1,5 times windspeed in 10 knots, but in 30 knots you will be sailing at 0,5 times windspeed. (if you`re upright.)